feat: channel balances move to spark at low fees #1768

Twin: synonymdev/bitkit-ios#1206
Refs: #1697

Problem or use case

Users who still have a Lightning channel from before Bitkit 3.1.0 have two Lightning balances once Spark ships in 3.2.0: the channel and Spark. Blocktank no longer sells channels and closes the existing ones progressively. A closed channel's balance goes to Savings, and moving it back to Spending then costs on-chain fees, Spark's deposit claim fee and 3 confirmations. Today the app pays from the channel first and falls back to Spark, so a channel balance only shrinks as the user spends it, and nothing tells the user any of this.

Proposed solution

• Move to Spark: one action that pays the channel's maximum sendable amount to the user's own Spark invoice over Lightning, showing the routing fee before the user confirms. That is the cheapest way to move the balance.
• Close afterwards: offer to close the emptied channel cooperatively, which returns the channel reserve (it cannot be sent over Lightning) to Savings.
• Post-update sheet: like the USDT intro sheet, shown once after the update to users who have a channel holding funds. It says Blocktank channels are being closed, recommends moving the balance to Spark as the lowest-fee option, and opens Move to Spark. It stays reachable afterwards from Spending or Lightning Connections while a channel holds funds.
• Design: an AI-made first version, then a Figma design and review by the designers, then polish.

Alternatives considered

• Do nothing and let Blocktank's closes return the funds to Savings. That costs the user on-chain and deposit fees to get them back to Spending.
• Close the channel first, then deposit to Spark. Same extra fees and a 3-confirmation wait.

Additional context

• A large balance may not route from Blocktank to Spark's provider in one payment. Handle failure with a smaller amount or a clear "amount too large" state.
• Start once the Spark PRs are ready for review; this builds on them.